Updated at 5:30 p.m. ET: LAS VEGAS -- Using a hammer as a weapon, a "complete stranger" with no significant criminal history allegedly attacked a family in their Las Vegas home, killing a woman and her daughter, in a brutal crime that left investigators both baffled and aghast.

Bryan Clay, 22, was arrested Friday in the April 15 rape and bludgeoning deaths of 38-year-old Ignacia Martinez and 10-year-old Karla Martinez. He had no connection to the family of five, Lt. Ray Steiber said Saturday.

"This was a complete stranger killing a mother and daughter and attacking the father," Steiber told The Associated Press. "I've been doing this (police work) 24 years, and you don't see cases like this. I can't even put this into words."

Nothing was taken from the blood-spattered house, and investigators were unsure of the motive for the attack.

"There's no rhyme or reason to why (it happened)," he said, adding Clay doesn't have a "significant" criminal history.

Clay also was booked in the beating and rape of a 50-year-old woman in the same west Las Vegas neighborhood hours before the slayings.

Steiber said he didn't know why two boys, 9 and 4, were spared in the home invasion attack that came to light when the older boy told school officials next day that his mother and sister were dead at home.

Arturo Martinez, 39, the husband and father, was critically injured in the attack and remains hospitalized with head injuries. He has been unable to talk to investigators. Both the mother and daughter were sexually assaulted, Steiber said.'

In the earlier attack, the 50-year-old woman was walking near an intersection when an assailant forced her into a nearby desert area and violently sexually assaulted her April 15. "(She was) chased, beaten and raped," Steiber said.

DNA results linked Clay to both attacks, investigators said. A baseball cap left behind by the woman's attacker turned out to be a key piece of evidence, KLAS-TV reported.

Authorities found the bodies of the girl and the mother in separate bedrooms. The two boys remained in the home for at least 24 hours with the bodies and their severely injured father, Steiber said.

Police were notified about the case the next day after the 9-year-old boy came to school and informed a counselor that his mom and sister were dead. The boys were placed in protective custody with the Clark County Department of Family Services after the attack, and police declined to say where they now are.

"They're safe and OK," Steiber said.

Police made the case a top priority as up to 60 investigators were involved. "Our goal was to get this guy into custody, and we weren't going to stop until we did," Steiber said.

Steiber said he didn't know if Clay had a lawyer, and attempts to reach a police spokeswoman were unsuccessful.

Clay was being held without bail Saturday pending an initial court appearance. He was booked on various charges, including murder, battery with a deadly weapon and sexual assault.